An in-depth look at screwworms and the USDA response to keep the problem from spreading in the U.S.

An adult female screwworm fly can lay up to 300 eggs
at a time. (USDA photo via Offrange)

 

Hollywood horror flicks can't hold a candle to the gruesomely parasitic screwworm. The flesh-eating larvae were eradicated in the U.S. nearly 60 years ago, only to reappear in South America and then migrate back into Mexico, where the Department of Agriculture has played hardball at the border, trying to keep the insidious pest from re-entering the states.

Luckily for cattle and other mammals, the screwworm, which is a type of blowfly's larva, isn't even a particularly effective parasite because it inevitably kills its host.

Female Cochliomyia hominivorax fixate on the smell of blood from any wound as a place to "lay their eggs — up to 300 at a time. These hatch out spined maggots that anchor themselves into flesh, then proceed to eat their way through it, sometimes deep into muscle," reports Lela Nargi of Offrange. "If it’s deep enough, the host animal cannot survive."

The USDA continues to use a multi-pronged approach to control screwworms:

• In May, U.S. Secretary of Agriculture Brooke L. Rollins suspended the import of live cattle, horses, and bison through U.S. ports of entry along the Mexican border, aiming to curtail entry possibilities while providing time to prepare and counter any screwworm infestations. While Rollins announced a phased reopening of the southern ports this July, the plan comes with heightened and constant monitoring.
• The plan includes the incremental release of thousands of sterile male screwworm producers with heavy monitoring by U.S. and Mexican scientists. The mass numbers of sterile screwworm-producing blowflies work to create a buffer by out-competing the fertile male flies. Since a female screwworm blowfly can only mate once in a lifetime, when she mates with a sterile male blowfly it insures that her eggs will never hatch -- no matter how many eggs she lays.
• Experts can also trap and measure the ratio of sterile to fertile male screwworm producers in different areas. Sterile releases aim to overwhelm the wild population, which scientists can also measure through traps.
• The USDA also employs teams affectionately known as "tick cowboys" who patrol borders on horseback, looking for any signs of illness in herds. By scouting for any cattle that appear distressed, the USDA can quickly detect, contain, and report any screwworm (or other) infestation.

An active screwworm infestation can be treated and cured, but like the worms themselves, it's "disgusting," Nargi writes. "The maggots have to be removed before the wound is cleaned and sutured." 

Screwworms do have a natural enemy -- fire ants. Nargi writes, "Unsurprisingly, no one is talking about dispersing more fire ants to eat the screwworms."

Experts are already hard at work helping livestock owners get prepared. Philip Kaufman, an entomologist at Texas A&M, "helped compile a new screwworm fact sheet that’s being distributed to ranchers," Nargi reports. "He isn’t prone to screwworm pessimism." He told her, "We are concerned, but we are not in a crisis. People are getting trained up and we’re gonna tackle this and solve it again."

Sharing any suspicious findings is key. Kaufman told Nargi, "Reporting is the number one thing people can do. If you ignore this, that fly population is just going to get bigger and bigger and cause more of a problem for everybody else. This is a community issue.”

Cows and people are susceptible to H5N1 avian flu; scientists are still learning how the disease moves

Big cats can contract H5N1.
(Photo by M. Foskett, Unsplash)

Amid outbreaks of H5N1 avian flu in birds and other wildlife species, the disease has now infected cows and humans. "Texas health officials reported that an individual who had been in contact with cattle has contracted H5N1 avian flu, only the second case ever recorded in the U.S.," reports Helen Branswell of STAT. "The individual's only symptom is eye inflammation — infection of the conjunctiva, the tissue surrounding the eye. . . .The infected individual was treated with the influenza antiviral oseltamivir, sold as Tamiflu."

Confirmed H5N1 outbreaks in cattle have been reported in Texas, Kansas and Michigan, alongside presumed cases in New Mexico and Idaho. "The virus doesn't kill the cattle, but milk production is lowered, and the animals' feeding is reduced," Branswell explains. Previously, cows weren't considered susceptible to H5N1, which is deadly for birds and has been contracted by numerous mammals, including big cats, bears, foxes, donkeys, goats and dogs.

Earlier versions of H5N1 have proved more deadly for humans, but as the disease has evolved, it "seems to trigger human infections less frequently than earlier versions of the virus did. And when human cases caused by this strain occur, they are typically mild," Branswell reports. "But people who have studied influenza — and this virus in particular — for years do not know what to make of its movement into so many different animal species."

Richard Webby, an influenza virologist who heads the World Health Organization Collaborating Center for Studies on the Ecology of Influenza in Animals at St. Jude Children's Research Hospital in Memphis, Tenn., "called the infections in cows 'a head-scratcher,' saying he would not have figured cattle to be on the list of animals susceptible to this virus," Branswell writes. Webby added, "​​This particular version of H5 is teaching us a number of things we thought we knew weren't right."